SBIR-STTR Award

Multi-rotors (e.g. quad-copters) typically have direct electric drive, where the electric motor shaft is directly coupled to the propeller shaft. The benefit of this configuration is simple and high fidelity control. But electric drive for vertical lift typically relies on lithium polymer batteries for energy storage, and battery specific energy is extremely low compared to internal combustion fuels; Gasoline has about a 15X advantage over rechargeable batteries and diesel has about an 18X advantage. Current unmanned multi-rotor aircraft do not have the endurance or payload capability to act in place of manned observatory platforms (rechargeable batteries deliver at most two hours of endurance for multi-rotor aircraft with no payload). However, frequency response requirements tend to prohibit direct drive from an internal combustion engine. Aurora proposes to develop a reformulated Miller Cycle engine in Series Hybrid Architecture for use in small unmanned vertical lift aircraft to combine the benefits of both direct electric drive and internal combustion engine technology. The reformulated Miller Cycle will also confront the fuel mixing issues associated with sUAS sized small engines.

A major market for vertical lift aircraft is in urban operations, primarily for police and electronic news gathering (typically a Bell 206 or a Eurocopter AS350). Manned systems are more costly to operate and have a much larger operational footprint than their unmanned counterparts. But the unmanned multirotor does not have the range and endurance to compete with the manned systems. Aurora Flight Sciences believes that the Passive Miller Cycle (PMC) Series Hybrid System is a viable way to achieve the range and endurance required to penetrate the manned vehicle market. The PMC, like the typical Miller Cycle, uses delayed intake valve timing that allows the expansion ratio to be greater than the compression ratio; reducing pumping losses and giving greater energy extraction. But the PMC does not use a positive displacement supercharger. The delayed intake valve closing also allows the PMC greater quench in the combustion chamber to confront the fuel droplet issue associated with small engines. The delayed valve timing also allows the generator in the hybrid system to be optimized for power generation while still being used as the engine starter. Based on the models developed in the Phase I program, Aurora will design, procure, and integrate the components required to demonstrate the Passive Miller Cycle (PMC) in a series hybrid architecture. The test system will be used to calibrate Phase I models and design a multirotor using the PMC hybrid system that will be able to perform police and news gathering missions.